Method of activating cds-photoresistors by heating in contact with cds admixed with cdo and cdse

ABSTRACT

A METHOD OF ACTIVATING AND/OR RECRYSTALLIZING A CDSPHOTRESISTOR IN WHICH A CDS BODY IS HEATED IN CONTACT WITH A POWDER MIXTURE OF CDS, CDSE AND CDO IN AN OXYGEN-FREE ATOMOSPHERE.

United States Patent U.S. Cl. 117201 22 Claims ABSTRACT OF THE DISCLOSURE A method of activating and/or recrystallizing a CdS- photoresistor in which a CdS body is heated in contact with a powder mixture of CdS, CdSe and CdO in an oxygen-free atmosphere.

The invention relates to a method of manufacturing CdS-photoresistors with CdS bodies which are subjected to a thermal treatment in contact with CdS.

In the known method, first a CdS- body is manufactured by compressing CdS powder or by sputtering or vapourdepositing CdS On a neutral support, for example, of ceramic material or glass. Doping of these bodies by copper and/or silver is carried out either by vapour deposition of these metals or by a thermal treatment in an atmosphere which contains the said materials. The CdS- bodies thus manufactured are then recrystallized and activated by a thermal treatment, the doping substances being incorporated in the newly-formed CdScrystals. After providing electrodes which are preferably formed by vapour-deposited metal layers, the CdS-photoresistor is ready for use. Of a universally applicable CdS-photoresistor the following properties are demanded, the simultaneous realisation of which in one resistor could so far he achieved only unsatisfactorily:

(1) A high dark resistance, i.e. a high electrical resistance at illumination intensity of zero;

(2) A low -value, i.e. a low ratio of a resistance variation occurring at a given variation of the illumination intensity to this variation of the illumination intensity. Normally 'y is stated for a variation of the illumination intensity between 0.4 lux and 25.6 lux.

(3) A favourable distribution of the spectral sensitivity;

and

(4) A high stability of the said properties in time, that is during the total lifetime of the photoresistor.

It is generally known to homogenize (recrystallize) and activate semiconductor crystals and semiconductor layers, particularly those from CdS, by heating them, while embedded in a powder which consists of the same or a related raw material with material properties which are desirable in the semiconductor to be treated, together with the powder at such a temperature that an exchange of material occurs between the semiconductor and the powder by thermal diffusion (German patent specification 919,727).

In order to obtain a particularly good recrystallisation of the CdS it was furthermore known toheat CdS-bodies in a HCl-containing atmosphere (DAS 1,139,588). However, it has been found that upon using the known above 3,598,645 Patented Aug. 10, 1971 method it is not possible, to realize the above desired properties simultaneously.

It is one of the objects of the invention to provide a method of manufacturing CdS-photoresistors which gives better results than the known method.

The invention is characterized in that, for activation and possibly recrystallisation, the CdS-bodies are heated in contact with a mixture of CdS, CdSe and CdO in an atmosphere containing at least temporarily free oxygen or an oxygen-supplying compound. This method provides the advantages that the photoresistors thus manufactured simultaneously show a high dark resistance, a low 7- value, a suitable distribution of the spectral sensitivity and a high stability of these properties during the lifetime of the photoresistors.

This is probably due to the fact that (1) due to the high partial pressure of CdS deviations from the stoichiometry in the CdS-bodies have disappeared in a short period of time, and thus a perfect CdS-body is formed;

(2) by the CdSe decomposing at the activating tempera ture, Cd is liberated, which dopes the CdS-body, i.e. forms photosensitive centers therein; and

(3) CdO, in the presence of S0 in the atmosphere, produces compounds (sulphites and sulphates) which make the photoresistor sta'ble.

Recrystallisation of the CdS-body may be performed simultaneously with the activation or, according to further embodiments according to the invention, prior to activation by heating in contact with CdS or by heating in a HCl-containing atmosphere. The contact with the said substances CdS, CdSe and CdO is preferably obtained by embedding the CdS-bodies in powders of these substances.

Doping of the CdS bodies is preferably carried out so that prior to recrystallisation copper and/or silver is vapour-deposited on the body and is then incorporated therein during the recrystallisation process. However, it is also possible to dope the CdS-bodies by the addition of copper or silver to the powder mixture in which recrystallisation takes place.

The invention will now be described in greater detail.

The CdS-bodies serving as bodies for the photoresistors to be manufactured can be produced by compressing CdS- powders to pellets of suitable size. Such compact bodies, however, have a rather large thickness, so that the danger exists that after a rather long recrystallisation and activation treatment the bodies are not homogeneous. Therefore, the CdS-bodies are preferably manufactured by spraying or vapour deposition of CdS on a neutral support, for example, a plate of a ceramic material. Usually a CdS-layer of approximately 5 m. to 10 am. thickness is vapour-deposited.

The doping of the CdS-bodies with copper and/or silver is effected either by vapour depositing said metals on the bodies or by admixing to the CdS-powder in which recrystallisation takes place, so that during the recrystallisation process the two above metals are incorporated in the CdS.

The further treatment of these CdS-bodies is carried out in two steps. First the vapour-deposited CdS-bodies are recrystallized either by heating in contact with CdS in which the bodies either lie on CdS-powder or are 3 buried in CdS-powder, or by heat ng in a HCl-containing atmosphere.

If the recrystallization is carried out in the CdS-powder, the powder may be mixed with at most 30% by weight and preferably 8% by weight of CdO.

The CdS-bodies are heated at a temperature between 350 C. and 650 C. preferably at 600 C. The duration of the heating is from 10 minutes to 60 minutes, preferably 45 minutes.

If the bodies are recrystallized in contact with CdS, this is carried out in a neutral atmosphere which, however, may temporarily contain SO or The activation of the recrystallized CdS bodies is carried out by heating bodies in contact with the mixture of CdS, CdSe and CdO. The CdS-bodies are preferably embedded in a mixture consisting of powders of these substances. The mixture may consist of 2 to 14 parts by weight of CdS, 1 to 6 parts by weight of CdSe and 1 part by weight of CdO, preferably of 7 parts by weight of CdS, 3 parts by weight of CdSe and 1 part by weight of CdO. The activation takes place at a temperature between 450 C. and 550 C. preferably at 480 C. The duration of heating is from 5 minutes to 70 minutes preferably 16 minutes.

The activation is first carried out in a neutral atmosphere, then, however, for at least one minute in an atmosphere containing S0 or 0 The neutral atmosphere may consist of N CO or A.

EXAMPLE I The doping substances are vapour-deposited on 8 ,um. thick CdS-layers provided on a support of a neutral ceramic material from a Ta-boat with 3 mgms. of Cu and 3 mgms. of Ag, which is arranged at a distance of 25 cm. from the CdS layers. For recrystallizing the CdS, the bodies are embedded in a powder consisting of 92% by weight of CdS and 8% by weight of CdO and heated for 45 minutes at 600 C. in an S0 atmosphere. For activation, the CdS-bodies are then again embedded in a powder which consists of 7 parts by weight of CdS, 3 parts by weight of CdSe and 1 part by weight of CdO. In this powder the bodies are heated at 480 C. for 16 minutes in an atmosphere consisting of N for 14 minutes succeeded by an atmosphere consisting of S0 for 2 minutes.

Electrodes consisting of indium and gold are then vapour-deposited on the CdS-bodies which are now photosensitive and the resistors are sealed hermetically in an envelope under dry air (H O content under 100 p.p.m.) The resistors thus manufactured have excellent properties. Their dark resistance lies at approximately 1M0, the 'y-value is approximately 0.65 and the stability in time is very good. For determining the stability the photoresistors are stored in complete darkness first for four hours at 60 C. and then for 20 hours at 25 C. and finally at 60 C. for four hours and an illumination intensity of 400 lux. Prior to and after every storage the resistance values are measured at 25 C. and an illumination intensity of 0.4 lux. In the photoresistors manufactured as described above, the average value of the variation is less than 7% of the value of the resistance.

EXAMPLE 2 CdS-layers, likewise 8 urn. thick, which, as described in Example I, are doped and recrystallized by embedding them in a powder which likewise consists of 7 parts by weight of CdS, 3 parts by weight of CdSe and 1 part by weight of CdO, and heating at 450 'C. for 16 minutes in an atmosphere consisting of N for 14 minutes and then consisting of 0 for 2 minutes.

The further treatment of the photoresistors is carried out as described in "Example I. The properties of the photoresistors thus manufactured are in agreement with those manufactured according to Example I.

4 EXAMPLE III In contrast with Examples I and III, CdS-bodies which, as described in Example I, are manufactured by vapourdeposition on a support and are vapour deposited with the doping metals Cu and Ag, are not embedded in powder which serves for recrystallisation and activation but are laid on the powder or provided at a small distance above the powder.

The recrystallisation then takes on or above a powder which consists of CdS alone. The CdS-bodies and the powder are for this purpose heated for 30 minutes in an N atmosphere, succeeded by heating for 10 minutes in an S0 atmosphere at 600 C. For activation a powder is used which consists of 5 parts by weight of CdS, 5 parts by weight of CdSe and 3 parts by weight of CdO on which powder, as described, the CdS-bodies are laid or provided above it. The CdS-bodies and the powder are heated at 480 C. for 12 minutes in an N -atmosphere, succeeded by heating at 480 C. for 4 minutes in an S0 atmosphere.

Then, as described in Example I, electrodes are vapour deposited on the CdS bodies and enclosed in envelopes; the properties of the photoresistors manufactured according to this example correspond to those which are manufactured according to Example I.

What is claimed is:

1. In the method of manufacturing CdS-photoresistors, the step of activating a CdS body by heating the body while in contact with a powder mixture of 2 to 14 parts by weight of CdS, 1 to 6 parts by weight of CdSe and 1 part by weight of 'CdO in an oxygen-free atmosphere.

2. A method as claimed in claim 1, wherein prior to activation the CdS-bodies are recrystallized by heating in contact with CdS powder.

3. A method as claimed in claim 2, wherein prior to activation the CdS-bodies are recrystallized by heating in a 'HCl-containing atmosphere.

4. A method as claimed in claim 1, wherein the CdS bodies are embedded in the powder mixture.

5. A method as claimed in claim 2, wherein a doping substance which is an element selected from the group consisting of copper and silver is vapour-deposited on the CdS-bodies prior to recrystallization.

6. A method as claimed in claim 2, wherein a doping substance for the CdS-bodies selected from the group consisting of copper and silver is added to the powder mixture in which recrystallization takes place.

7. A method as claimed in claim 2, wherein the CdS- powder in which recrystallization takes place is mixed with at most 30% by weight of CdO powder.

8. A method as claimed in claim 7, wherein the powder is mixed with 8% by weight of CdO powder.

9. A method as claimed in claim 2, wherein recrystalllzation is carried out at a temperature between 350 C. and 650 C.

10. A method as claimed in claim 9, wherein recrystallization is carried out at a temperature of 600 C.

11. A method as claimed in claim 9, wherein recrystallization is carried out for a period of time of from 10 to 60 minutes.

12. A method as claimed in claim 11, wherein recrystallization is carried out for a period of time of 45 minutes.

13. A method as claimed in claim 2, wherein recrystallization is carried out in a neutral atmosphere.

14. A method as claimed in claim 2, wherein recrystallization is carried out in an atmosphere containing S02 or 02.

15. A method as claimed in claim 1, wherein the powder mixture contains 7 parts by weight of CdS, 3 parts by weight of CdSe and 1 part by weight of CdO.

16. A method as claimed in claim 1, wherein activation is carried out at a temperature 'betwen 450 C. and 550 C.

17. A method as claimed in claim 16, wherein activation is carried out at a temperature of 480 C.

18. A method as claimed in claim 16, wherein activation is carried out for a period of time of from 5 to 70 minutes.

19. A method as claimed in claim 18, wherein activation is carried out for a period of time of 16 minutes.

20. A method as claimed in claim 1, wherein activation is carried out in an atmosphere which is first neutral and then contains S0 or 0 for at least 1 minute.

21. A method as claimed in claim 20, wherein the neutral atmosphere consists of N CO or A.

22. A method as claimed in claim 1, wherein the CdS- UNITED STATES PATENTS 5/1966 Rotschild 117201 4/1968 Chamberlin et a1. 117-201 0 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 11762 

